The secret lives of whale sharks finally unveiled!

For decades, marine scientists have watched whale sharks (Rhincodon typus), the world’s biggest fish, from boats and while snorkelling. They could only catch glimpses of the world’s largest fish gliding through tropical waters.

It turns out that what they have observed until now was only the trailer. A new study published in the journal Marine Biology reveals their krill chases and complex behaviour. Researchers mounted cameras on these charismatic marine animals near Ningaloo Reef in Western Australia and discovered that the giants do not mostly feed at the surface, as was previously assumed. Instead, they are calculating foragers that dive deeper than 70 metres, shifting between different feeding strategies depending on what they find.

Spying on the giants

The lives of whale sharks below the surface have mostly remained a mystery. These magnificent creatures live in tropical oceans, where food is often sparse, and hence they must constantly adapt to survive. With climate change affecting them, food is becoming increasingly scarce. That is exactly why it is important to understand their feeding strategies. However, following them into deep oceans using conventional observation methods proved impossible.

Once they dive, they vanish from sight. Though electronic tracking tags tell researchers where whale sharks go, they offer no insight into what they are actually doing down there. That is when scientists decided to attach camera-equipped tags to the dorsal fins of whale sharks. The cameras captured high-resolution footage from the sharks’ own perspective. “The camera provided footage of the front half of the shark, along with its own point of view.

It is as close as we can get to seeing the ocean through their eyes,” the researchers wrote in The Conversation.After 24 hours, the tag detached automatically, leaving no trace on the animal and allowing researchers to retrieve the footage. This footage helped researchers understand what happens below the water’s surface at a tropical coastal aggregation site. “Our work has revealed new behaviours and, for the first time, provides a more comprehensive picture of whale shark foraging strategies throughout the water column, from the surface to the bottom of the sea,” the researchers said.

What do whale sharks do when no one is watching?

The researchers wanted to see what these massive fish were doing when nobody was watching. “We could finally address the question: ‘What do whale sharks do when no one is watching?’ Filling these research gaps is essential so we can help these majestic, yet endangered, filter-feeding animals,” the researchers noted. Along with the footage, they reviewed over 100 scientific publications detailing whale shark behaviour.

Scientists catalogued 36 distinct feeding strategies, ranging from slow, energy-efficient gliding with open mouths near the seafloor to vigorous chases after dense krill swarms at the surface.“We discovered whale sharks use a mix of several feeding behaviours throughout the water column and along the seafloor, up to 70 metres deep at our study site,” the researchers revealed. Previous studies suggested that whale sharks predominantly feed at the surface; however, the new findings challenge that assumption.

'Low-effort' snacking

The researchers observed a range of feeding behaviours. Whale sharks did not constantly hunt. Instead, they spent the majority of their time performing low-effort feeding behaviours, such as sinking to the seafloor with an open mouth (slow gliding feeding). They also engaged in vigorous swims after krill swarms, their preferred prey at the surface (active surface feeding when food is abundant).The researchers noted that whale sharks spend most of their time performing six low-effort feeding behaviours.

This means they essentially graze through areas where food is scarce. This works like snacking from a pantry while deciding what to cook for dinner. They nibble for most of the time and switch into an aggressive hunting mode the moment they encounter higher-density prey.“We also found that whale sharks have a much wider variety of foraging strategies compared to other filter-feeding megafauna, such as basking sharks and whales.

This could help explain how they manage to thrive in tropical waters where prey is limited and patchy,” the researchers said.The study was limited to one location at one time: juvenile whale sharks gathered near the coast at Ningaloo Reef, Western Australia. “If researchers can attach more cameras to whale sharks at aggregation sites around the world, we will hopefully discover new insights and continue to grow our understanding of this iconic species,” they added. The researchers are thrilled that they are beginning to understand the secret lives of the biggest fish on Earth.

“And from what we have seen, they are the ultimate snackers of the sea.”